The invention relates to the field of 4-cycle engines. More specifically, the invention relates to methods, apparatus, and systems for distinguishing between compression and exhaust cylinder strokes for 4-cycle engines.
Modern ignition systems rely on accurate crankshaft position calculations to determine a corresponding piston position in a cylinder, which is necessary to determine an optimal time to fire a spark plug for the cylinder. It is well known in the art to calculate crankshaft position of 2-cycle engines by observing two pulse trains generated by sensors that detect, for example, flywheel teeth in a ring gear. The first pulse train has the same angular frequency as the engine, providing a single pulse per engine revolution at the Top Dead Center (TDC) position of a piston in a predetermined cylinder. The second pulse train provides a number of pulses per engine revolution equal to the number of flywheel teeth in the ring gear. Using these two pulse trains as references, the ignition system can calculate a crankshaft position, corresponding piston position, and determine proper ignition timing.
For 4-cycle engines, however, a spark plug for a cylinder is fired every other engine revolution, as the piston for the cylinder alternates between compression and exhaust strokes. Therefore, to determine proper ignition timing, it is also necessary to distinguish between compression and exhaust strokes. Traditionally, ignition systems for 4-cycle engines require a third pulse train to distinguish between compression and exhaust strokes. This third pulse train may be generated by a cam reference sensor that provides a pulse every other engine revolution, possibly at a Top Dead Center (TDC) position of a known cylinder on its compression stroke.
FIG. 1 illustrates a block diagram of an exemplary ignition system with an ignition controller 111 to distinguish between exhaust and compressions strokes according to a method that utilizes a camshaft reference sensor.
Flywheel sensor 100 detects magnet 101 on flywheel 102, generating pulse train 108, which has the same angular frequency as the engine, and provides a single pulse per revolution of the engine (1/REV) at the Top Dead Center point of a known piston. Ring gear sensor 103 senses the teeth on ring gear 104, generating pulse train 109, the Position Indicating Pulses (PIP), which has an angular frequency that is equal to the angular frequency of the first pulse train multiplied by the number of teeth in the gear wheel. Thus, the Position Indicating Pulse has an angular resolution equal to 360xc2x0/number teeth.
In order to distinguish between compression and exhaust strokes, the camshaft sensor 105 detects magnet 106 on camshaft 107, generating pulse train 110 that provides a pulse every other revolution (1/2REV) on Top Dead Center of the compression stroke of a known cylinder. Modifying the crankshaft to provide for camshaft sensor 105 and magnet 106 is expensive. Further, the additional wiring and interface circuitry for camshaft 105 adds cost and complexity to the ignition system.